1. Technical Field
The present invention relates generally to wireless communication systems and, more particularly, to a multi-standard transmitter system and method for noise reduction through phase modulation.
2. Description of Related Art
Wireless communication devices, such as cellular telephones, are widely used as a replacement for conventional telephone systems. One advantage of the wireless communication devices is their portability. The user can operate the wireless communication devices from virtually any point on earth. Since component size, weight, and power requirements of the wireless communication device can detrimentally affect portability, they are important factors that directly impact its utility.
For communication to occur, signals are transmitted from and received by components of the wireless communication devices. Transmitters, either separate or part of a transceiver, handle transmission tasks for the wireless communication device. Transmitters typically accept complex digital baseband signals to be transmitted. These complex digital baseband signals are internally generated within the wireless communication device. The transmitters subsequently perform forms of modulation, frequency up-conversion, digital-to-analog conversion and power amplification of the baseband signals.
Digital-to-analog conversion is an important aspect for transmitters since it has the potential of producing a great deal of signal noise. Conventional approaches to digital-to-analog conversion include using particular kinds of digital-to-analog converters (DACS) with a relatively high number of operational bits to perform the digital-to-analog conversion. For instance, some transmitters use 10-bit or 12-bit DACs. Other conventional approaches use DACs with fewer number of operational bits to reduce costs, but are forced to alleviate the additional noise caused by the lower bit DACs by using elaborate filtering.
The trade-off analysis between the amount of noise generated by low-bit DACs versus the expense associated with high-bit DACs become even more complex for transmitters configured for a multi-standard communication device such as a multi-standard cellular telephone. In particular, aspects of cellular telephones standards related to multiplexing of simultaneous telephone calls impact greatly the implementation of DACs in a multi-standard environment. In general, multiplexing is performed with cellular telephone systems either with a frequency division multiple access (FDMA) method, a time division multiple access (TDMA) method or a code division multiple access (CDMA) method.
With the FDMA methods, individual simultaneous cellular telephone calls are assigned different frequencies within a given frequency band. As shown in FIG. 1, a frequency band will have a bandwidth, Bf with individual carrier frequencies, F1 through Fn. With the FDMA method, a pair of individual carrier frequencies supports one simultaneous cellular telephone call in which one carrier frequency of the pair handles communication from mobile stations to base stations and the other carrier frequency of the pair handles communication from base stations to mobile stations.
TDMA methods also use transmission frequency bands having individual carrier frequencies, however, the individual TDMA carrier frequencies are further divided by time based multiplexing so that a pair of TDMA carrier frequencies can support multiple simultaneous telephone calls. For instance, as shown in FIG. 2, a pair of TDMA carrier frequencies can support a multitude of simultaneous telephone calls since each carrier frequency is divided into numerous time segments wherein one carrier frequency of the pair is used for uplink communication and the other carrier frequency of the pair is used for downlink communication.
CDMA methods differ from the FDMA and TDMA methods in that the CDMA methods use transmission frequency bands in which no individual carrier frequencies are designated for particular telephone calls. Instead, and individual telephone call can be distributed throughout a particular frequency band between frequencies FA and FB as shown in FIG. 3.
Global System for Mobile Communications (GSM) is a cellular telephone communication standard that uses a particular form of TDMA multiplexing in which the individual carrier frequencies of a frequency band are spaced in 200 kHz intervals as shown in FIG. 4. Under the GSM standard, as shown in FIG. 5, for each carrier signal having a particular carrier frequency, fsignal, a first amount of power is measured in a 30 kHz bandwidth area centered on the carrier frequency, fsignal, and a second amount of power is measured in a 30 kHz bandwidth area centered on a test frequency ftest, that is 400 kHz away from the carrier frequency, fsignal. According to the GSM standard, the second amount of power should be at least 60 decibels below the first amount of power.
Standards that use CDMA technology are less demanding regarding noise requirements compared with the GSM standard. Second generation CDMA technology uses carriers having wide transmission frequency bands, such as the 1.23 MHz frequency band, shown in FIG. 6. Third generation CDMA technology uses Wide CDMA (WCDMA), which uses carriers having transmission frequency bands of five MHz to 15 MHz such as shown in FIG. 7. The CDMA and WCDMA standards regarding noise requirements tend to focus on areas of noise reduction outside of the transmission frequency bands of the carriers. As a result, the CDMA and WCDMA standards are easier to comply with regarding noise requirements than the GSM standard since the GSM standard addresses noise from each of the numerous carrier signals within its frequency band. For instance, the GSM standard for operating with a 890 MHz to 915 MHz uplink frequency band and 935 MHz to 960 MHz downlink frequency band has 124 pairs of individual carrier signals on separate frequencies, each with separate noise requirements regarding filtering or noise reduction. Consequently, conventional approaches in implementing the GSM standard typically use high-bit DACs. As an example, in a typical situation involving CDMA or WCDMA, an 8-bit or 9-bit DAC with simple post-conversion filtering would suffice. In contrast, in a typical situation involving GSM, a 10-bit or 12-bit DAC with more elaborate post-conversion filtering would be required. In this case, for GSM, if an 8-bit or 9-bit DAC were to be used, even more extensive and elaborate filtering would be required.
Conventional approaches that address use of cellular telephones with more than one communication standard, such as with GSM and WCDMA, struggle with the challenge of meeting the noise requirements of both standards as it relates to digital-to-analog conversion of signals internal to the cellular telephone. Some conventional approaches use a low-bit DAC for the CDMA noise requirements and an elaborate system of switchable filters when GSM is required. Other conventional approaches use dual communication paths having separate DACs directed to each communication standard. For instance, in a first communication path within a cellular telephone transmitter, one or more 8-bit or 9-bit DACs would be used for the CDMA or WCDMA technology whereas in a second communication path in the cellular telephone transmitter, one or more 10-bit or 12-bit DACs would be used for the GSM technology. Unfortunately, the conventional approaches have been relatively expensive and complicated to implement.
Accordingly, there is a significant need for a system and method for a transmitter in a wireless communication device, such as a cellular telephone, to operate under more than one communication standard without the need for switchable filters or multiple communication paths to address noise requirements of the multiple standards with particular respect to the one or more DACs used in the transmitter. The motivation for such a need include reduction of the costs and complexity associated with conventional approaches toward multi-standard transmitters. The present invention provides this and other advantages that will become apparent from the following detailed description and accompanying figures.